1. B. N. Bershad, T. E. Anderson, E. D. Lazowska and H. M
B.N. Bershad, T.E. Anderson, E.D. Lazowska and H.M. Levy Lightweight Remote Procedure Call ACM Transactions on Computer Systems, Vol.8, No.1, February 1990, 37-55
Lanfranco Muzi
PSU – April 21th, 2005

2. Presentation outline What is LRPC?
O.S. communication – most common case
LRPC execution model
Performance of LRPC
Conclusions

3. What is LRPC?
A communication facility designed and optimized for communication between protection domains in the same machine
Target: small-kernel operating systems
separate components placed in disjoint domains
messages used for interdomain communication

4. O.S. communication - the common case Cross-domain vs cross-machine
Vast majority of calls do not cross machine boundaries

5. O.S. communication: the common case Size and complexity of calls
~70%
95% of calls to 10 procedures
75% to 3 procedures!
Most frequent case: less than 200 bytes
Vast majority of calls: small, simple arguments/results

6. O.S. communication: the common case Performance (I)
“In existing RPC systems, cross- domain calls are implemented in terms of the facilities required by cross-machine ones”

7. O.S. communication: the common case Performance (II)
“Theoretical minimum”: 2 traps and 2 context switches
Overhead: stub, message buffer, access validation, message transfer (queues), scheduling (independent threads), context switches…

8. LRPC execution model Client binds to server before making first call
Call causes kernel trap
Kernel: validate caller, create linkage, dispatch client’s thread to server
Thread executes in server
Control (and thread) back to kernel and caller

9. Binding - 1 Different from RPC “Import” call from client via kernel
Server provides Procedure Descriptor List, used by kernel to allocate A-stacks and create linkage record
At completion, kernel returns to client a Binding object and A-stack list

10. Binding - 2 A-stack: Read-write and shared by both domains
Linkage record:
Records caller’s return address + current SP
Binding object:
Client’s key to interface (must be presented to kernel at every call)

11. Calling Client stub: Kernel: Pop A-stack off the queue, push arguments
Put A-stack address, Binding Object and procedure identifier into registers
Trap
Kernel:
Validation etc.
Find E-stack in server domain and update user thread stack pointer
Reload CPU virtual memory registers with those of the server domain

12. Optimization 1 Argument copying
RPC
A-stack
LRPC
A-stack: Read-write and shared by both domains

13. Optimization 2
Stub: choice between cross-domain or –machine in 1st line
Validation: only on call (linkage is in thread’s control block on return)
Scheduling in RPC: different threads in each domain signalling at each other (block client, dispatch in server…)

14. Optimization 3 Multiprocessors:
Reduced lock contention: each A-queue has its own lock – no other locking required
Reduced context-switch overhead: Domain caching on idle processors – move to idle CPU avoiding context switch

15. Performance of LRPC - 1 Average on 100,000 cross-domain calls
LRPC/MP uses domain caching
TAOS uses RPC
Taos/LRPC ~= 3!

16. Performance of LRPC - 2
The “context switches” cost is what domain caching helps reduce
Single processor: LRPC adds only 48ms

17. Call throughput on a multiprocessor
Performance of LRPC - 3
Call throughput on a multiprocessor
Domain caching disabled
RPC scales poorly due to lock contention, which is very limited in LRPC

18. Conclusions
LRPC adopts an optimized approach to the most common case in O.S. communication
Performs well on the majority of PC between address spaces on the same machine
Avoids unnecessary synchronization, kernel-level thread management and data copying
LRPC performs better than RPC in the common case, and scales better on multiprocessors
Authors conclude that it is a good communication alternative for small-kernel O.S. (further developments?)

19. References
B.N Bershad et al. “Lightweight remote procedure call” ACM Trans. Comput. Syst. 8,1 (Feb.1990) 37-55
A.D. Birrel et al. “Implementing remote procedure calls” ACM Trans. Comput. Syst. 2,1 (Feb.1984) 39-59
B.N. Bershad et al. “User-level interprocess communication for shared memory multiprocessors” ACM Trans. Comput. Syst. 9,2 (May1991)
A.S. Tanenbaum “Modern Operating Systems” 2nd ed. Prentice Hall (RPC: )
Silberschatz, Galvin, Gagne “Operating system concepts” 6th ed., John Wiley & Sons (RPC: )
“Remote procedure calls” Pznan University of technology – Institute of computing Science